Prior art evaporator defrosting methods and apparatus include such concepts as initiating defrost as a function of time, as a function of air flow through the evaporator coil, as a function of frost thickness, as a function of the differential between coil temperature and the temperature of a served space, and the like. If defrost is based upon some arbitrary value, such as time, then in certain instances defrosting may occur too often, or not often enough, resulting in inefficient operation.
If defrost is based upon need, some sort of additional sensor is usually required, which adds to the cost and maintenance of the defrost apparatus.